Antibiotic-resistant bacterial infections are a major and increasing problem in US hospitals. Among antibiotic resistant bacterial pathogens, Vancomycin-Resistant Enterococcus (VRE) is emerging as a leading cause of blood-stream infections, particularly in patients receiving treatment for cancer or following allogeneic hematopoietic stem cell transplantation (allo-HSCT). Our studies demonstrated that VRE dominates the intestinal microbiota of patients being treated with antibiotics. We have also demonstrated that introduction of a normal microbiota into VRE-dominated mice eliminate VRE carriage. The overall goal of this application is to identify commensal bacteria in mice and humans that confer resistance to intestinal colonization by VRE and to identify mechanisms of VRE-specific colonization resistance. We have established a bank of bacterial strains that, in aggregate, confer resistance to VRE, and will use these strains to identify strain combinations that provide complete colonization resistance. Our second aim is to characterize mechanisms of VRE clearance by protective commensal microbes and to test the hypothesis that innate immune activation and microbiota-mediated modification of the metabolome synergize to reduce VRE density in the gut. Our preliminary data demonstrate that secondary bile salts strongly inhibit VRE growth and our experiments will determine whether loss of secondary bile salts enables VRE to achieve a state of domination in the cecum. The third aim is to transfer banked fecal microbiota from allo-HSCT patients who were susceptible or resistant to VRE intestinal domination into germ-free mice to identify bacterial strains from the human microbiota that confer resistance to VRE colonization and/or domination. These studies are addressing the growing problem of antibiotic resistance in an era where the prospects for further antibiotic development are dim. Identifying commensal bacteria that provide resistance to infection by antibiotic- resistant pathogens and determining mechanisms of VRE clearance may lead to more effective approaches to reduce infections and patient-to-patient transmission.